1. Field of the Invention
The present invention relates to techniques of removing foreign substances such as dust, and particularly relates to a technique of removing foreign substances such as dust which adhere to a surface of an optical member arranged in front of an image pickup element in an image pickup apparatus such as a digital still camera.
2. Description of the Related Art
In general, image pickup apparatuses such as digital still cameras capture objects and convert images of the objects into electric signals. In such an image pickup apparatus, an imaging light beam is received using an image pickup element, a photoelectric conversion signal output from the image pickup element is converted into image data, and the image data is stored in a recording medium such as a memory card. Examples of the image pickup element include a CCD (charge coupled device) sensor and a CMOS (complementary metal oxide semiconductor) sensor.
In such an image pickup apparatus, an optical low-pass filter or an infrared absorption filter is arranged on an object side of the image pickup element. If foreign substances such as dust adhere to surfaces of the filters, portions to which the foreign substances adhere are shown as dark spots in a captured image, and accordingly, appearance of the image is deteriorated.
In particular, in a digital single-lens reflex camera having a replaceable lens, since mechanical operation portions such as a shutter and a quick-return mirror are arranged in the vicinity of an image pickup element, foreign substances such as dust generated due to operations of the operation portions may adhere to a surface of an image pickup element and a surfaces of a filter. In addition, when the lens is replaced by another lens, foreign substances such as dust may enter from an opening of a lens mount into a camera body and may adhere to the inside of the camera body.
To avoid this problem, U.S. Pat. Nos. 7,006,138 and 7,215,372 disclose techniques of removing foreign substances such as dust adhering to surfaces of anti-dust filters which allow imaging light beams to be transmitted to object sides of image pickup elements by vibrating the filters using piezoelectric elements.
In such a technique, preferably, a wide vibration width is obtained by vibrating an anti-dust filter in a resonant frequency using the piezoelectric element so that the foreign substances such as dust are effectively removed. However, since the resonant frequency is determined in accordance with an external form, a thickness, and a property value of the anti-dust filter, for example, different anti-dust filters have different resonant frequencies due to variation-generating factors such as a processing accuracy and a manufacturing process.
Here, a camera disclosed in U.S. Pat. No. 7,006,138 has a piezoelectric element which applies vibration to a peripheral portion of a circular anti-dust filter (anti-dust optical member), and has an electrode which is used to detect a vibration state of the optical member and which is provided on the piezoelectric element separately from an electrode used for a driving operation. Then, a voltage of the electrode is monitored so that a resonant frequency is detected, and the optical member is vibrated using the resonant frequency so that the foreign substances such as dust adhering to a surface of the optical member are removed.
Furthermore, as with the case of U.S. Pat. No. 7,006,138, a camera disclosed in U.S. Pat. No. 7,215,372 has a piezoelectric element which applies vibration to a peripheral portion of a circular anti-dust filter (anti-dust optical member). Furthermore, the camera includes an electrode which is used to detect a vibration state of the optical member and which is provided on the piezoelectric element separately from an electrode used for a driving operation, and a voltage of the electrode is monitored. Then, the voltage of the electrode is compared with a normal voltage stored in advance. As a result of the comparison, when it is determined that the vibration state is abnormal, an operation of removing dust is not performed.
As a circuit used to drive a piezoelectric element, a half-bridge circuit is generally used. The half-bridge circuit has a simple circuit configuration in which an input voltage (power supply voltage or booster voltage) is directly used to drive the piezoelectric element. Alternatively, an H-bridge (full bridge) circuit may be employed. Although a configuration of the H-bridge circuit is more complicated than that of the half-bridge circuit, a driving voltage twice an input voltage can be applied to the piezoelectric element. Therefore, when identical driving voltages are applied to respective piezoelectric elements using the H-bridge circuit and the half-bridge circuit, the H-bridge circuit only requires the input voltage half of the input voltage of the half-bridge circuit. Accordingly, withstand pressure standards of circuit elements may be reduced and miniaturization of the circuit is attained.
However, although the configuration of the vibration detection described above is appropriately employed in the half-bridge circuit serving as the driving circuit for the piezoelectric element, when the configuration is employed in the H-bridge circuit, the vibration of the optical member is not properly detected.